So it turn out that when i plug it to PC, i can see it as usb massive storage and system immediately wants to format Clip, but when i hit format button it shows that device has only 4mb capacity and when i try to format Clip, it says that it can't be formated.

That means the flash memory is either blank or burned out. Since you didn't do anything that would have erased it, and were apparently getting random file corruption before, my guess is the flash chip died.